Varnish is

* a high-performace HTTP accelerator designed for content-heavy
  dynamic web sites.
* designed for maximum performance on modern hardware and modern
  operating systems. Varnish takes full advantage of modern virtual
  memory systems and of advanced operating system features such as
  kqueue, accept filters, sendfile etc. where available.
* open source, published under a two-clause BSD license.

Homepage:
https://varnish-cache.org/
